my-pmpro-change-error-message.php

<?php
/**
 * A simple example to show you how to change an error message for checkout on Paid Memberships Pro.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function pmpro_change_error_message( $translated_text, $text, $domain ) {
	if ( 'paid-memberships-pro' !== $domain ) {
		return $translated_text;
	}

	switch ( $text ) {
		case 'That email address is already in use. Please log in, or use a different email address.':
			$translated_text = __( 'That email address is already in use. Please<a href="/login/">log in</a>, or use a different email address.', 'paid-memberships-pro' );
			break;
	}

	return $translated_text;
}
add_filter( 'gettext', 'pmpro_change_error_message', 20, 3 );